feat(video): add post-processing brightness filter for windowed mode support

[bobtista]

Implements GenTool-style brightness adjustment using post-processing filters instead of hardware gamma ramps. This fixes brightness control in windowed mode while maintaining fullscreen support.

Note: When testing, I was able to change brightness settings (Full screen), but screenshots look identical. I know my screen was brighter, I'm not sure how the screenshots don't show it...

Addresses #1554

Changes

Architecture

• Replaced hardware gamma ramp approach with shader-based post-processing
• Implemented ScreenBrightnessFilter (Pixel Shader 1.1) and ScreenBrightnessFilterFixedFunction (fallback)
• Integrated with existing W3DShaderManager filter system

Features

• Supports GenTool's extended range (-128 to +256)
• Range -128 to 0: Darken
• Range 1-128: Lift black level linearly
• Range 129-256: Lift brightness while preserving black level (gamma adjustment)

Technical Details

• Created W3DBrightnessFilter.h/cpp with filter implementations
• Added brightness.pso pixel shader (Pixel Shader 1.1 assembly)
• Wired options menu brightness slider to filter
• Removed windowed mode restriction in W3DDisplay::setGamma()
• Filter automatically enabled on display initialization

Testing

• [] Brightness adjustment in windowed mode
• Brightness adjustment in fullscreen mode
• Various brightness values (-128, 0, +128, +256)

Backward Compatibility

• Option.ini "Gamma" key unchanged
• Slider range unchanged (0-100)
• Existing settings respected
• Old gamma ramp code preserved